CODEWITHSUNDEEP
shellunix
Quentin Le Bévillon
Quentin Le Bévillon

Reputation: 98

shell builtin with redirection

We are working on a shell (school project). We don't understand a behaviour. Why builtins are not acting when redirected ?

like

cd - | command

does not change the directory.

or

export NAME=VALUE | command

does not create the variable.

Thank you.

Upvotes: 3

Views: 238

Answers (2)

Petr Skocik
Petr Skocik

Reputation: 60067

Links of a pipeline are run in forked subshells.

In bash, you can print the PID of the current process with $BASHPID, so something like:

self(){ echo $BASHPID; } ; self ; self >&2 | self; self

should give you something like:

12849
12851
12852
12849

with the middle two PIDs being different than the first and the last (the mother shell) (in some shells, the first or the last link is run in the mother shell, but not in bash).

Changing the current directory or exporting a value in a subshell won't affect the parent shell in any way whatsoever.

Upvotes: 3

the.Legend
the.Legend

Reputation: 686

I think you're confusing pipes '|' with semicolons ';' when building toolchains. Pipes are for passing stdout of one command into stdin of another. Semicolons are for running one command after another in the same shell.

Thus in order to enter some directory and list its content you should do this:

cd somedir; ls

usually in Linux all processes of a pipeline are started at the same time in subshells, with their streams appropriately connected, and managed by the scheduler together with all other processes running on the machine. Their outputs are reconnected through buffers and synchronized

That's why you don't see changes of export command because variable stayed in child's subshell.

Upvotes: 0

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